Discriminating Depositional Habitat and Provenance of Mudrock Through Use of Spectral Gamma Ray

Paxton, Stanley T.¹, Anna M. Cruse², James O. Puckette², Michael Aufill² (1) 1009 Woodcrest, Stillwater, OK (2) Oklahoma State University, Stillwater, OK

Spectral gamma ray (SGR) provides detailed information about sedimentation, depositional environment, and provenance of mudrock. For the present study, SGR data were sampled at locations throughout North America from outcrops, well logs, and cores. Collectively, based on work of previous investigators, the mudrocks represent a range of ages and depositional settings. To prepare the data for analysis, K (%), U (ppm), and Th (ppm) values were converted to API gamma-ray units through use of a standard petrophysics equation. Next, the data were normalized in terms of relative contributions to total calculated API and results plotted on a ternary diagram with K, U, and Th at each apex.

The analysis suggests that SGR of mudrock deposited under anoxic and/or euxinic conditions is dominated by U (organic-rich, pyrite-bearing, and finely laminated). The abundance of U in black shale (and hence, the strength of the gamma-ray signal) is attributed to the duration of starved sedimentation on the ancient seafloor (which favors diffusion and fixation of U in the substrate). SGR measurements that plot close to the U pole on the triangle represent organic-rich “condensed” sections. In contrast, mudrock that is relatively enriched in K and/or Th is interpreted to have accumulated under more oxic depositional conditions. K-rich mudrock is suggestive of derivation from relatively unweathered provenances with sedimentation rates >10 cm/ky. Th-rich mudrock is characteristic of a provenance or depositional setting that has undergone deep weathering or leaching to produce a “residuum”. In theory, some Th-rich mudrock may represent an "organic-poor" condensed section.